The lugworm (Arenicola) — A study in adaptation

Abstract

The lugworm is adapted, not so much to an environment as to a way of life. The ordinary, day-to-day life of the lugworm is reviewed, as a basis for the discussion of its adaptations. The worm generally lives in a burrow of characteristic form, and for most of the time it carries out a regular rhythmic sequence of movements (the Normal Cyclical Pattern), determined by certain spontaneous pacemakers and serving to integrate its various necessary activities. The N.C.P. is seen in its most typical form when the burrow is covered over by water. It can be modified to incorporate a method of aerial respiration under low-tide conditions, or as a periodic “testing” mechanism if the animal should be trapped in a limited volume of foul water. The N.C.P. is part of the worm's innate organisation, and clearly adapted to its way of life. In special circumstances, the worm carries out alternative behaviour sequences, apparently unrelated to the N.C.P. The worm's body is divided into regions by local modifications of the metameric segmental plan. The functional significance of some of these regional differentiations is clear. The tail is a means of communicating with the sand surface in comparative safety, and the parapodia at the two ends of the trunk are divergently specialised in accordance with different types of movement. Certain other differentiations (distribution along the body of gills and nephridia, variation in the course of the efferent branchial vessels) can perhaps be explained as adapted to the headward water stream which the worm drives through its burrow, but in these cases the explanations, though superficially plausible, are unsupported by positive experimental evidence. The properties of Arenicola haemoglobin are considered as an example of physiological adaptation. The peculiarities of the oxygen dissociation curve are hard to understand if we assume that the worm is trying to do with its haemoglobin the same kind of things that we do with ours, and one may seriously doubt whether the pigment ever plays a part in oxygen transport. Recent suggestions that the haemoglobin serves, among other functions, to protect the worm against two poisons—oxygen excess and hydrogen sulphide—offer explanations of the peculiarities of the oxygen dissociation curve, of the considerable individual variations in haemoglobin concentration, and of the great development of haematopoietic tissues in the body.